As is well known, a metallic or non-metallic magnetic material loses its magnetic properties upon being heated to a temperature above its Curie temperature and regains its magnetic properties upon being cooled to a temperature below its Curie temperature. Various devices have heretofore been proposed which utilize this phenomenon. Examples of such devices are described in United States Letters Patent No. 3,149,246 issued to W. P. Mason on Sept. 15, 1964, United States Letters Patent No. 3,445,740 issued on May 20, 1969 to G. G. Merkl and United States Letters Patent No. 3,743,866 issued on July 3, 1973 to A. Pire.